N-Channel JFET Characteristics Curve

The transconductance characteristics curve of a JFET transistor is the the curve which shows the graph
of the drain current,
ID verses
the gate-source voltage, VGS.

The ratio of change in drain current,
∆ID, to the change in gate-source voltage, ∆VGS,
is the transconductance, gm. This is what this characteristic curve serves to show.

The transconductance curve, as for all semiconductor devices, is nonlinear, for most of the curve,
meaning changes to VGS
do not directly increase or decrease drain current, ID.

Below is the characteristic curve for an N-Channel JFET transistor:

How an N-Channel JFET Works

An N-Channel JFET turns on by taking a positive voltage to the drain-source terminal of the transistor
and a gate-source voltage,
VGS,
between 0V and -4V. It
shuts off by taking in a negative gate-source voltage, VGS, below -4V.

Regions of the Characteristics Curve

The Regions that make this characteristic curve are the following:

Cutoff Region- This is the region where the JFET transistor is off, meaning no drain current, I
D flows from drain to source.

Ohmic Region- This is the region where the JFET transistor begins to show some resistance to the
drain current, Id that is beginning to flow from drain to source. This is the only region in the curve
where the response is linear.

Saturation Region- This is the region where the JFET transistor is fully operation and maximum current,
for the voltage, VGS, that is supplied is flowing. During this region, the JFET is On and active.

Breakdown Region- This is the region where the voltage, VDD that is supplied to the drain
of the transistor exceeds the necessary maximum. At this point, the JFET loses its ability to resist current
because too much voltage is applied across its drain-source terminals. The transistor breaks down and current flows
from drain to source.